It is a principal concern of the cost to manufacture a power supply in selecting between power supply types to be used in particular application and the components selected to construct them. Since integrated circuit technology has advanced such that a majority of the complex switching mode circuits can be integrated on a single chip, switching mode power supplies have become cost competitive with much simpler linear power supplies.
A prior art flyback power supply with a voltage regulator is schematically shown in FIG. 1. In a power supply circuit 10, a full-wave bridge rectifier 12 accepts an AC power from a set of power input terminals 14, and a DC power is therefore supplied to a transformer 18 in association with a filter capacitor 16. The transformer 18 comprises a primary winding 20 and a pair of secondary windings 22 and 24. A capacitor 28 is charged through the winding 22 and a diode 26 such that a power supply output voltage is provided at a set of output terminals 30 of the power supply circuit 10. On the other hand, a capacitor 34 is charged through the winding 24 and a diode 32 to provide a feedback voltage, which is delivered to a pulse width modulator (PWM) 40 through a voltage divider consisting of resistors 52 and 54. An end of the winding 20 is connected to a high voltage switching transistor 38 in a regulator circuit 36, which is turned on/off under the control of a signal from the PWM 40. The control signal from the PWM 40 to a gate of the switching transistor 38 oscillates at a frequency with a duty cycle such that a preset output voltage is maintained between the set of output terminals 30 of the power supply circuit 10.
A high voltage power start up transistor 42 is used to start up the regulator circuit 36. When power is turned on, the transistor 42 is turned on and the capacitor 34 is charged to provide a line 50 with internal low voltage power. Once the voltage on the line 50 reaches a threshold voltage, the PWM 40 functions, and when the voltage on the line 50 reaches a specific threshold voltage, a transistor 46 is turned on. As a result, the threshold voltage from a voltage source 48 is connected to a gate of the transistor 42, and thus the transistor 42 is turned off. As shown in FIG. 1, a resistor 44 is connected to a drain of the transistor 42.
To overcome the inconveniences caused by the high voltage pins of a chip and the high voltage itself, Balakrishnan disclosed in U.S. Pat. No. 5,014,178 a high voltage power start up transistor directly connected to a drain of a high voltage switching transistor, while in U.S. Pat. No. 5,313,381 a feedback control signal and a bias supply voltage combined at a single pin with a metal-oxide-semiconductor (MOS) transistor, such as that proposed in U.S. Pat. No. 4,811,075 to Eklund. However, complicated circuits and expensive manufacturing processes are presented in these prior arts. In addition, it is desired a further integration and excellent performance for an SMPS circuit.